1. Field of the Invention
The present invention relates to a liquid crystal display element applicable to a reflective type or a transflective type liquid crystal display, a method for producing the liquid crystal display element and a color display device.
2. Discussion of the Background
A liquid crystal display element has widely been used for a portable information device or the like because it is thin and light in weight. As such liquid crystal display element widely used, there are an active matrix liquid crystal display element using active elements such as thin-film transistors and a STN liquid crystal element as a simple matrix type liquid crystal display element which responds an effective value of voltage. Use of the active matrix liquid crystal element is currently in a main stream because it exhibits a high display performance. However, it is unsuitable for such a field of use that a variety of design or custom-made articles are required, because manufacturing steps are complicated and expensive and a plurality of masks are required for manufacturing a kind of display apparatus. Accordingly, the simple matrix type liquid crystal display device is often used in such field of use.
A liquid crystal display element assembled to an information system is often required to have a color display from the viewpoints that a display is easy to see and has a large amount of information to be displayed.
In order to realize a color display, a system in which a color filter is provided in a liquid crystal display cell is used. However, in the liquid crystal display element provided with a color filter, the transmittance of light is decreased due to the color filter, and when an additive color mixture method is used, it is necessary to divide a single pixel into three portions for three basic colors of red, green and blue (R, G, B) with the result that the numerical aperture decreases substantially.
For instance, the brightness of red in displaying red is expressed by (the transmittance of liquid crystalxc3x97the transmittance of a red color filterxc3x97numerical aperture/3). However, an obtainable value does not reach 3% of light entering into the liquid crystal cell. Therefore, a light source such as a backlight is provided at a position such as a rear face of the liquid crystal display element to form a transmission type liquid crystal display device. Further, for assurance of an attractive display in such construction, it is necessary to operate the light source all the time.
The characteristics required for the color display device used for a portable information device reside in the following two points: (a) a color display of multi-gray shades of almost the full colors can be provided regardless of a change of the lightness of the surroundings and (b) power consumption is low.
In the transmission type color liquid crystal display device used widely for a book type personal computer at the present time, the brightness of about 100 cd/cm2, which is an appropriate lightness in an ordinary office environment, is standard. However, the portable information device is nowadays used under various environments such as a large meeting room (which has a fixed illumination and a user""s desire is not often satisfied), the outdoors, at night and so on.
On the other hand, since the visual performance of human is generally adaptable to an illuminance of outer light, there exists the optimum brightness of an object to be observed in response to the illuminance of an outer light.
For example, the illuminance of an outer light is about several ten luxes under a dark environment, and a human vision is adaptable to darkness. In this case, when the brightness of an object to be observed is too light, a stimulation to the optic nerve is strong and the observer feels fairly fatigue. Accordingly, the brightness of the backlight should be decreased to about several cd/m2-50 cd/m2. Further, under an environment of a high illuminance such as the outdoors in a fine weather, the illuminance of an outer light exceeds 20,000 luxes, and accordingly, a human vision is adaptable to lightness. In this case, when the brightness of an object to be observed is too dark, visibility to a display remarkably decreases. Therefore, the brightness of the backlight should be as high as about several hundred-1,000 cd/m2.
Accordingly, in order to allow the use of it in any environment, it is necessary to achieve the highest brightness of not less than several hundred cd/m2 and to conduct a brightness control of 100:1 or more.
In conducting such brightness control, a large amount of consumption power and an accessory circuit are needed. In short, the transmission type color display device belonging to the conventional technique is not suitable for portable information devices from either the viewpoint of consumption power or the viewpoint of an increased number of elements for the accessory circuit.
On the other hand, since the portable information devices are generally of a type of battery-driven, there is a demand of lowering an amount of power to be consumed in order to prolong a driving time. Therefore, there is proposed, for the portable information devices, to use a reflective type color liquid crystal display device without using a backlight while an amount of power to be consumed can substantially be reduced.
In the reflective color liquid crystal display device, however, there is a problem that the brightness is very low because it does not have a backlight and both an incident light and a reflected light are passed through the color filter.
Namely, in the conventional reflective type color display device without having an auxiliary light source, although visibility to a display can sufficiently be assured under an environment of a high illuminance such as the outdoors in a fine weather, it is almost impossible to recognize a display under an environment of darkness. Accordingly, it is difficult to use the reflective type display device under such a change of environment.
Further, the transmission type liquid crystal display device generally uses a cold cathode ray tube as a backlight. Accordingly, the color filter is generally so designed as to utilize sufficiently light from the cold cathode ray tube. However, when light from a light source other than the cold cathode ray tube is passed through such color filter, an achromatic color can not be obtained at the time of mixing colors, and good color development cannot be obtained. For example, under a daylight condition, a color obtainable by the color filter at the time of mixing colors is yellowish green.
Further, JP-A-7-28010 and JP-A-8-179125 disclose a liquid crystal display element using a transflector.
However, the inventors of this application has revealed through their studies that even though the construction of the conventional transmission type display device is simply changed according to specifications for a transflective type, a sufficient performance of display can not be obtained.
It is an object of the present invention to provide a liquid crystal display element which can be used under various environments while a good color-developing performance is maintained, a method for producing the liquid crystal display element and a color display device.
In accordance with the present invention, there is provided a liquid crystal display element comprising a color filter having a plurality of colored portions each having a different spectral color and a reflector, wherein the color filter is so adapted that transmitted light from a standard C light source is substantially achromatic and the transmittance of visible light by the standard C light source is 30-65%.
Further, there is provided the above-mentioned liquid crystal display element wherein a colored portion having a spectral color of the color filter is formed to cover a pixel of a driving aperture, which corresponds to the colored portion having a spectral color of the color filter.
Further, there is provided a color display device comprising a display element having a color filter, a backlight and a transflector disposed between the display element and the backlight wherein the transmittance of visible light by a standard C light source, of the filter is 30-65%, and the transmittance T(%) and the reflectance R(%) of the transflector satisfy Formula 1:
T/(T+R)xe2x89xa60.4. . . .xe2x80x83xe2x80x83Formula 1
The liquid crystal display element of the present invention is provided with a reflector. The reflector may be a transflector. In the present invention, xe2x80x9ctransflectionxe2x80x9d means xe2x80x9csemi-transmission and semi-reflectionxe2x80x9d and therefore, the transflector, as a single member, possesses both a light transmitting property and a light reflecting property.
In the present invention, there is obtainable a liquid crystal display element which provides easy observation for a display under various environments by utilizing efficiently an outer light.
Further, in the color filter used for the present invention, transmitted light from a standard C light source is substantially achromatic. Here, xe2x80x9csubstantially achromaticxe2x80x9d implies that chromaticity coordinates x and y obtainable after light from the standard C light source has transmitted through the color filter, are in a range of white on a chromaticity diagram (mainly in a range of generally elliptic shape of [x=0.27, y=0.27]-[x=0.37, y=0.37]) shown in a reference Figure in JIS Z8110.
With such measures, the features of the reflective type display element utilizing efficiently an outer light can be taken while a good color-developing performance is obtainable.
Generally, a color filter used for a display device which effects a color display of nearly full colors, is provided with a plurality of color portions each having a different spectral color. In this case, xe2x80x9ctransmitted light of a color filterxe2x80x9d implies the transmitted light, in the whole color filter, resulted when the colored portions having a spectral color respectively produce color lights and the color lights are mixed.
When the transflector is used for the color display device of the present invention, a color tone of the color filter may be determined in consideration of a color tone of the backlight so that chromaticity coordinates x and y of light obtained after light from the standard C light source has passed through the color filter are deviated from the above-mentioned range of white.
In this case, however, since a transflector having a relatively high reflectance in comparison with the transmittance in the present invention is used, an outer light contributes largely to the brightness of a display. Accordingly, in fact, it is preferable that the chromaticity coordinates x, y of light after light from the standard C light source has passed through the color filter are within the above-mentioned range of white (transmitted light from the standard C light source is substantially achromatic).
Further, in the present invention, the transmittance of visible light by the standard C light source through the color filter is to be 30-65%, particularly, 30-55%. Since the color filter of the present invention has a relatively high visible light transmittance. Accordingly, when it is used for a reflective type liquid crystal display element, an outer light can sufficiently be utilized for a display.
Further, in the present invention, it is preferable that any of the colored portions having a spectral color of the color filter is formed to cover a pixel of a driving aperture wherein the pixel corresponds to a color portion having a spectral color of the color filter. Here, the pixel of the driving aperture means a portion in which a column electrode and a row electrode, which are arranged in a matrix form, overlap.
With such measures, the all area of the driving aperture is usable for transmission of light whereby a degree of utilization of an outer light is improved and brightness in a display device can be improved.
The colored portions having each a spectral color to form the color filter can be positioned in three ways as follows. Namely, color portions each having a spectral color for constituting the color filter are arranged to adjoin without any gap; there is a mixed color portion between adjacent colored portions each having a spectral color for constituting the color filter, and there is a black mask between adjacent colored portions each having a spectral color for constituting the color filter.
In a case that a colored portion having a spectral color of the color filter is formed to cover a pixel of a driving aperture, wherein the pixel corresponds to the color portion having a spectral color of the color filter, it is preferable to form a liquid crystal display element in a mode of transmitting light at the application of a voltage (i.e., a normally black mode). It is because the mode of transmitting light at the application of a voltage can suppress leakage of light from an area other than pixels at the time of interruption of light, in comparison with a mode of interrupting light at the time of the application of voltage (i.e., a normally white mode).
Further, in the present invention, when the transflector is used so that both a display performance obtained by transmitted light and a display performance obtained by reflected light of an outer light can be obtained, it is preferable that the transmittance T(%) and the reflectance R(%) of the transflector satisfy the above-mentioned Formula 1. With such measures, a display by transmitted light and a display by reflected light can be used in balance whereby a display adaptable naturally to a change of environment can be obtained. Here, xe2x80x9ca display by transmitted lightxe2x80x9d means a display obtained by utilizing a backlight attached as an accessory to a color liquid crystal display device, and xe2x80x9ca display by reflected lightxe2x80x9d means a display effected by utilizing an outer light.
Description will be made as to conditions of Formula 1.
In the transflector, as the reflectance is higher, a display which utilizes mainly reflected light is presented while a loss in the operation of the backlight is large. On the other hand, when the reflectance is low, a degree of utilization of an outer light is low.
In order to improve a color tone in utilizing transmitted light, the color purity of the color filter is usually increased as a result of which a structure of lowering the transmittance is used. However, such structure is unsuitable for a display using reflected light. Further, transmitted light takes a route in which the light passes once from a backside where the backlight is disposed to a front side. However, reflected light takes a route in which the light is incident from a front side, passes through the inside of the display device, is reflected by the transflector, and passes again through the inside of the display device, i.e., the light passes twice (reference to FIG. 18). Accordingly, the optimum condition of the color filter is different between the transmitted light and the reflected light.
Generally, a reflectance of transflector is expressed by R(%), a transmittance of transflector is expressed by T(%) and a transmittance of visible light by a standard C light source through a color filter is expressed by Y(%). Further, when a brightness of backlight is expressed by xcex1(cd/r2), an illuminance of outer light irradiated onto a surface of an object to be observed is expressed by xcex2(Lx), a directivity of light source is expressed by LSn, and a directivity of reflective member is expressed by Rn, a brightness of display is given by the following Formula 2:
xcex1xc2x7(T/100)xc2x7(Y/100)xc2x7C1+xcex2(LSn)xc2x7Rnxc2x7(R/100)xc2x7(Y/100)2xc2x7C2(cd/m2 )xe2x80x83xe2x80x83Formula 2
where LSn and Rn as directivity indication coefficients are respectively values corresponding to n obtained when brightness distributions of a diffusion characteristic and a reflection-diffusion characteristic are expressed in terms of cosn(xcex8), and they take a range of 1 less than n less than ∞. When n=1, there is no directivity which expresses perfect diffusion and perfect diffusion-reflection characteristics. Further, when n=∞, they show the strongest directivity which expresses perfect collimate light and perfect mirror-surface-reflection characteristics.
Further, C1 represents practically the maximum transmittance of liquid crystal and C2 represents practically the maximum transmittance of liquid crystal in consideration of the directivity of reflected light. Further, T and R are relative values wherein T+Rxe2x89xa6100%.
From this Formula, the characteristics of display by T, R and Y are largely classified into the following (A) to (D).
(A) When T=large, R=small and Y=small, a display by transmitted light is attractive and a degree of utilization of the backlight is high. However, a degree of utilization of an outer light is low.
(B) When T=large, R=small and Y=large, a display by mainly transmitted light is obtainable, however, the chromaticity is poor. A degree of utilization of reflected light is intermediate.
(C) When T=small, R=large and Y=small, a display by transmitted light is not attractive, and a degree of utilization of the backlight is low. A degree of utilization of an outer light is intermediate.
(D) When T=small, R=large and Y=large, a display by reflected light is intermediate, and a degree of utilization of the backlight is intermediate. A degree of utilization of an outer light is high.
According to the knowledge of the inventors, in a transflective type liquid crystal display element, a lightness required by transmitted light with use of the backlight is fairly smaller than that of a case of using a transmission type display device, and a brightness of ⅓ or lower is sufficient in comparison with a numerical value of 100 cd/m2 in a standardized use of a note type personal computer. Since a component of reflected light of an outer light can be utilized even in an ordinary environment of room, there is little problem in suppressing lightness obtained by transmitted light.
On the other hand, as described before, it is preferable that the transmittance of the color filter is relatively large in order to increase the brightness of a display by reflected light. From the above, in the transflective type display device, it is preferable to satisfy each relation as in formula 3 and formula 4.
T/(T+R)xe2x89xa60.4xe2x80x83xe2x80x83Formula 3
30xe2x89xa6Y less than 65xe2x80x83xe2x80x83Formula 4
As to Y, it is preferable to satisfy a relation in Formula 5.
30 less than Y less than 55xe2x80x83xe2x80x83Formula 5
Further, it is in particular preferable to satisfy conditions as in Formula 6 and Formula 7.
T/(T+R)xe2x89xa60.35xe2x80x83xe2x80x83Formula 6
35 less than Y less than 50xe2x80x83xe2x80x83Formula 7
Further, in order to sufficiently make use of the characteristics of the transflective type, it is preferable that T/(T+R) is 0.05 or more. In order to obtain good visibility in any environment, it is most preferable that T/(T+R) is 0.15 or less.
When these conditions are satisfied, a display in response to a change of environment can be obtained, and at the same time, a display of lower power consumption rate than the conventional transmission type is possible. For instance, an example that a transmission type display device of 100 cd/m2 is modified to a transflective type according to the above-mentioned way of thinking wherein the lightness at the time of transmitting light is 10c/m2, is taken.
In this case, when it is estimated that Y=45%, T=30%, R=70% and the ratio of consumption power by the backlight in the transmission type is 90%, the consumption power can be reduced to 70% in comparison with the transmission type. This means that the color liquid crystal display device of the present invention can achieve in good balance a response to a change of environment and the requirement for reducing consumption power.
In this viewpoint, a component of transmitted light and a component of reflected light are balanced, and a brightness by transmitted light is brought to such a condition that consumption power can be minimized in a case of a minimum necessary brightness in an environment of use.
For example, it is preferable that the brightness by transmitted light is 50 cd/m2 or less in the application of the display device to a personal information device.
Further, it is further preferable to be 30 cd/m2 or less.
Further, since a higher visibility is required for usage such as a car-mounted type, the brightness by transmitted light should be 120 cd/m2 or less. Further, as a further preferable condition, 80 cd/m2 or less is exemplified. Even in these examples, the brightness is about ⅓ as much as the conventional transmission type, and a requirement of lower consumption power can be met.
A desired member can be used as to the directivity of reflected light. However, when a transflector is used, it is preferable to use such reflector having a certain degree of directivity. As luminous intensity distribution characteristics at the time of reflection, a gain of 1.5-10 is a preferred range. At the same time, it is preferable to use a backlight having substantially the same directivity.